Condensed Consolidating Financial Information - Condensed Consolidating Statements of Comprehensive Income (Detail) - CAD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Net income	$ 545	$ 622	$ 436	$ 348	$ 984	$ 510	$ 480	$ 431	$ 1,951	$ 2,405	$ 1,599
Net (loss) gain in foreign currency translation adjustments, net of hedging activities									(60)	24	18
Change in derivatives designated as cash flow hedges									38	19	(2)
Change in pension and post-retirement defined benefit plans									(449)	80	(434)
Other comprehensive (loss) income before income taxes									(471)	123	(418)
Income tax recovery (expense) on above items									169	(65)	96
Equity accounted investments									0	0	0
Other comprehensive (loss) income									(302)	58	(322)
Comprehensive income									1,649	2,463	1,277
Consolidating Adjustments and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									(2,772)	(3,547)	(2,098)
Net (loss) gain in foreign currency translation adjustments, net of hedging activities									0	0	0
Change in derivatives designated as cash flow hedges									0	0	0
Change in pension and post-retirement defined benefit plans									0	0	0
Other comprehensive (loss) income before income taxes									0	0	0
Income tax recovery (expense) on above items									0	0	0
Equity accounted investments									(121)	237	447
Other comprehensive (loss) income									(121)	237	447
Comprehensive income									(2,893)	(3,310)	(1,651)
CPRL (Parent Guarantor) [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									1,951	2,405	1,599
Net (loss) gain in foreign currency translation adjustments, net of hedging activities									0	0	0
Change in derivatives designated as cash flow hedges									0	0	0
Change in pension and post-retirement defined benefit plans									0	0	0
Other comprehensive (loss) income before income taxes									0	0	0
Income tax recovery (expense) on above items									0	0	0
Equity accounted investments									(302)	58	(322)
Other comprehensive (loss) income									(302)	58	(322)
Comprehensive income									1,649	2,463	1,277
CPRC (Subsidiary Issuer) [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									1,969	2,367	1,566
Net (loss) gain in foreign currency translation adjustments, net of hedging activities									(479)	318	149
Change in derivatives designated as cash flow hedges									38	19	(2)
Change in pension and post-retirement defined benefit plans									(455)	82	(443)
Other comprehensive (loss) income before income taxes									(896)	419	(296)
Income tax recovery (expense) on above items									171	(66)	99
Equity accounted investments									423	(295)	(125)
Other comprehensive (loss) income									(302)	58	(322)
Comprehensive income									1,667	2,425	1,244
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									803	1,180	532
Net (loss) gain in foreign currency translation adjustments, net of hedging activities									419	(294)	(131)
Change in derivatives designated as cash flow hedges									0	0	0
Change in pension and post-retirement defined benefit plans									6	(2)	9
Other comprehensive (loss) income before income taxes									425	(296)	(122)
Income tax recovery (expense) on above items									(2)	1	(3)
Equity accounted investments									0	0	0
Other comprehensive (loss) income									423	(295)	(125)
Comprehensive income									$ 1,226	$ 885	$ 407